IGF-1 and GH interpretation — why serum GH tells you nothing

Serum GH is useless for monitoring growth hormone status. IGF-1 is the 24-hour integrated signal that actually reflects GH output. Here's how to interpret it, age-adjust it, and use it to track peptide protocols.

If you are running a GHRH/GHRP stack — CJC-1295, ipamorelin, tesamorelin, sermorelin, or any growth hormone secretagogue — the single most common interpretation mistake is staring at a serum GH number and trying to decide whether the protocol is working. It is not working or not working based on that draw. The draw tells you almost nothing. This article walks through why, what to look at instead, how to age-adjust your targets, and how to read IGF-1 trajectory like the integrated signal it actually is.

Why serum GH is not the monitoring tool

Pituitary growth hormone secretion is pulsatile. Healthy adults release GH in roughly 6–12 discrete bursts per 24 hours, with the largest pulses clustering during slow-wave sleep in the first half of the night. Between pulses, circulating GH falls back toward 0.0–0.5 ng/mL. During a pulse it can spike to 5–30 ng/mL or higher. The plasma half-life of GH is approximately 15–20 minutes, so by the time a single random draw lands in the tube, you are sampling a near-random point on a sawtooth waveform.

A morning fasted GH of 0.2 ng/mL does not mean you are GH-deficient. A mid-afternoon GH of 12 ng/mL does not mean your peptide stack is hyperdosed. Both numbers are compatible with completely normal physiology and both are compatible with pathology — you cannot tell which from a single timed draw. Even formal GH stimulation testing (arginine, insulin tolerance, glucagon, macimorelin) requires multiple timed samples and a provoked secretory response, which is well outside the scope of routine bloodwork.

IGF-1 as the integrated 24-hour signal

Insulin-like growth factor 1 is produced primarily in hepatocytes in response to GH binding the hepatic GH receptor. Its half-life in circulation, bound mostly to IGF-BP3 in a ternary complex with the acid-labile subunit, is roughly 12–15 hours. Because of that long half-life, plasma IGF-1 reflects the integrated effect of every GH pulse over the previous day or two. It does not bounce around with the underlying GH waveform — it averages it.

That is exactly the property you want from a monitoring marker. Drawn fasted or non-fasted, morning or afternoon, IGF-1 will not vary by more than a few percent within an individual on a stable protocol. Day-to-day biological variability is roughly 5–10 percent, well below the change you should expect to see from a working GHRH/GHRP stack. If your IGF-1 has moved 60 ng/mL up after eight weeks on CJC-1295/ipamorelin, that is signal, not noise.

Age-adjusted ranges — why the lab range is not your range

IGF-1 falls steadily across adult life. A 25-year-old in the 50th percentile sits around 220 ng/mL. A 45-year-old in the 50th percentile sits around 130 ng/mL. A 65-year-old in the 50th percentile sits around 110 ng/mL. The lab will print a single age-adjusted reference range next to your value, but the way you should think about it is: where do I sit on the percentile curve for someone two decades younger than me?

This is the actual reason people run GH-stimulating peptides — not to push past the upper limit of a 25-year-old's range, but to restore a 45-year-old back to a 25-year-old's median. If you are 45 and your baseline IGF-1 is 130 ng/mL and you push it to 220 ng/mL on a CJC/ipamorelin stack, you have not "elevated" anything in any meaningful sense. You have moved one age cohort to the median of a younger one. That framing matters when you are deciding whether 240 ng/mL feels too high.

What actually drives IGF-1 up and down

Five variables move IGF-1 independently of any peptide you inject. Miss them and your protocol attribution will be wrong.

Protein intake. Hepatic IGF-1 production requires substrate. Sustained protein intake below roughly 0.8 g/kg/day suppresses IGF-1 regardless of GH stimulation. Cohorts on long-term protein restriction or extended fasting routinely run IGF-1 30–40 percent below age-matched eating controls. If you are running a GHRH peptide while doing a low-protein cut, the peptide will look weaker than it is.

Sleep architecture. Roughly 60–70 percent of total daily GH output occurs during the first two slow-wave sleep cycles. Chronic sleep restriction or fragmented sleep (untreated apnea, late alcohol, blue light, late caffeine) cuts that endogenous output and pulls IGF-1 down. A peptide stack on a four-hour-sleep schedule will underperform.

Insulin sensitivity. Hepatic GH receptor expression and post-receptor signaling are insulin-sensitive. Insulin-resistant phenotypes (visceral adiposity, NAFLD, T2D) show GH resistance — normal or elevated GH pulses, blunted IGF-1 response. This is also why incretin-class agents matter: see retatrutide vs tirzepatide, where improving insulin sensitivity raises IGF-1 indirectly by restoring hepatic GH responsiveness.

Sex steroids. Testosterone increases GH pulse amplitude and raises IGF-1; this is one of the cleanest signals in the literature. Men on optimized testosterone (mid-to-upper reference) typically run 30–50 ng/mL higher IGF-1 than men with low total testosterone, all else equal. If you are stacking testosterone replacement and a GHRH peptide, you cannot cleanly attribute IGF-1 movement to either one in isolation. See the testosterone panel for ranges. Estrogen, particularly oral estradiol, does the opposite via first-pass hepatic effects — it suppresses hepatic IGF-1 production.

Thyroid status. Subclinical hypothyroidism blunts IGF-1. Treating it can raise IGF-1 measurably without any GH-axis intervention.

Protocol monitoring on peptides

Expected IGF-1 trajectories on common GH-stimulating peptide protocols, assuming adequate sleep, protein, and stable sex hormones:

• CJC-1295 (no DAC) + ipamorelin, typical doses, sub-Q nightly. Expect IGF-1 to rise 30–80 ng/mL over 6–8 weeks. Plateau at 8–12 weeks. Larger movements indicate either very low baseline or supraphysiologic dosing.
• Tesamorelin, the only GHRH analogue with a specific approved indication, drives more substantial IGF-1 elevation, typically 60–120 ng/mL over 12 weeks at standard dosing. It also reduces visceral adipose tissue independently of weight, which is why it gets used off-label in metabolic protocols.
• Sermorelin, shorter-acting, smaller IGF-1 effect, typically 20–40 ng/mL elevation. Often the right tool when you want a gentler axis push.
• MK-677 (ibutamoren), oral ghrelin mimetic, produces the largest IGF-1 elevations of the group — frequently 100–150 ng/mL — but with the most off-target effects (water retention, appetite, insulin resistance).

Draw IGF-1 at baseline, 6–8 weeks, and 12 weeks. Same lab, same assay. Do not chase week-to-week noise.

Upper boundary — what the evidence says about IGF-1 above 300 ng/mL

Adult IGF-1 sustained above roughly 300 ng/mL is the threshold where epidemiologic signals get noisier. Observational data link upper-quartile IGF-1 to small but non-zero increases in certain solid-tumor incidence (colorectal, prostate, breast). The effect sizes are modest, and reverse causation and confounding remain live questions, but the directional signal is consistent enough that pushing IGF-1 well above 300 ng/mL chronically deserves a deliberate justification.

Acute side effects correlate with similar territory: water retention, paresthesias, carpal tunnel symptoms, mild glucose intolerance, joint stiffness. These are dose-dependent, reversible on dose reduction, and a useful clinical signal that you have overshot.

A reasonable target for a long-term optimization protocol is 200–280 ng/mL — the upper-normal band of a younger cohort's median, well clear of the 300 watch zone.

IGF-BP3 — when to add it to interpretation

IGF-BP3 carries roughly 75–80 percent of circulating IGF-1 in the ternary complex. In most adults on a normal axis it tracks IGF-1, and the ratio (IGF-1 / IGF-BP3, often expressed in molar terms) is stable.

Order it when the IGF-1 number does not match clinical context. A high IGF-1 with a low IGF-BP3 suggests increased free IGF-1 fraction, which can signal more biologic activity than the total number implies. A low IGF-1 with normal IGF-BP3 sometimes points to assay or sampling artifact rather than true low GH activity. For most peptide users on a stable stack with congruent total IGF-1 trends, you do not need IGF-BP3 routinely.

The GH/IGF-1 disconnect

IGF-1 is a near-perfect proxy for GH activity in healthy adults. Several conditions break the proxy:

• Liver disease. Hepatic synthetic dysfunction lowers IGF-1 even when GH output is normal or elevated. Cirrhotic patients commonly have low IGF-1 and high GH.
• Hypothyroidism. Reduced hepatic GH receptor expression blunts the IGF-1 response. Correct thyroid first.
• Severe caloric or protein restriction. Substrate-limited liver cannot translate GH signal into IGF-1, regardless of how aggressive your peptide protocol is.
• GH resistance (Laron-type, acquired in chronic inflammation, sepsis, advanced renal disease). GH receptor signaling is impaired; high GH coexists with low IGF-1.
• Estrogen — particularly oral. First-pass hepatic suppression of IGF-1 production. Transdermal estrogen has minimal effect.

Putting it together

Stop looking at serum GH. Draw IGF-1 at baseline and at 6–8 and 12 weeks on protocol, same lab, same assay, fasted or non-fasted (it does not matter much). Read the result against the percentile of an age cohort 10–20 years younger than you, not against the lab's adult reference range. Target 200–280 ng/mL as your optimization band. Pay attention to the variables that move IGF-1 outside the peptide axis — protein, sleep, insulin sensitivity, sex steroids, thyroid — before attributing movement to the peptide itself. Add IGF-BP3 only when the IGF-1 number does not fit the clinical picture. Treat sustained values above 300 ng/mL as a signal to dose-titrate down rather than a goal to chase.

That is the entire framework. Pulse hormone, integrated signal, age cohort, confounders, dose ceiling. The number on the IGF-1 line is where the real information is.